Abnormal results on a full blood count (FBC) can cause significant angst. Yet, there is no such thing as a ‘normal range’. Reference ranges are derived from thousands of samples taken from healthy people. They vary with ethnicity, gender and age. The reference range is the middle 95% (two standard deviations from the mean) thus excluding the top and bottom 2.5% of healthy people.
Results falling just outside the accepted reference range may be due to statistical variability. The sample run multiple times may produce slightly varying results. Biological variability (test results can vary during the day) and age (FBC parameters may fall with age) influence results. We quote Caucasian ranges, which may not be applicable to other ethnicities.
The further removed a result is from the reference range, the more likely to be truly abnormal. Abnormalities in more cell lines can be more important. Is the defect new or recurrent, and over how long? Long-standing single aberrations without clinical features or consequences are less likely significant. Is it progressive or fluctuating? Are there morphological abnormalities? Is there a reasonably identifiable cause? What is the clinical context? In asymptomatic patients with low pre-test probability, the rate of false positive abnormal results will be high, and borderline results do not need investigation.
The total white cell count (WCC), itself, is much less important than the individual white cells. The lower limit of the neutrophils, lymphocytes, monocytes, eosinophils and basophils adds up to much less than the lower limit for total WCC. Similarly, the upper limits also are much higher than the upper limit for total WCC. This is because the ranges were done separately for all.
Rarely does a person have all cells at the lower (or upper) limit. So, if the individual cell counts are normal, the total cell count is not important.
Specifics
Mild neutropenia is common (e.g. viral infection, autoimmune disorders, drugs, liver disease and hypersplenism, B12 or folate deficiency, hypothyroidism, ethnic variation). Haematologists will rarely investigate a count above 1.0 without immature cells, other FBC defects, or clinical problems. Recurrent bacterial sepsis is the risk, and usually only under 0.5 for a long time, and even then, uncommon.
Mild neutrophilia is commonly seen in infection, inflammation, acute stress (e.g. acute myocardial infarction (AMI)), heavy exercise, drugs (lithium, steroids), pregnancy, and smoking.
Mild lymphopenia is common and non-specific. Haematologists rarely investigate if isolated without associated features. Almost any sickness can cause mild lymphopenia.
Lymphocytosis is due to acute infection, acute stress response, (e.g. AMI) smoking, or lymphoproliferative disorders. With persisting significant lymphocytosis immunophenotyping can be done, but usually not at the first presentation. Isolated lymphocytosis <5.0 does not usually need investigating.
Monocytosis is seen in chronic infections or inflammation (e.g. Crohn’s, Rheumatoid) and primary blood disorders. Investigation may be warranted if persistently above 5.0.
Eosinophilia is common in allergy problems. Investigation may be warranted if above 5.0 as primary marrow disorders are possible.
Basophilia is rarely seen and occurs in myeloproliferative disorders. Other cell abnormalities will be evident.
Thrombocytopenia is common and can be due to blood collection difficulty, viral infection, alcohol, Idiopathic thrombocytopenic purpura (ITP), drugs, pregnancy, autoimmune disorders, and primary marrow disorders. Above 100 is rarely worrying. Between 50 and 100 is usually safe whilst 30-50, though usually safe, needs investigation. Bleeding risk increases between 30 and 50 and requires referral. Under 10 carries major spontaneous bleeding risk and warrants hospitalisation.
Thrombocytosis is often reactive (e.g. infection, inflammation), from blood loss (including menorrhagia), iron deficiency, post trauma and less commonly myeloproliferative disorders. Usually below 1000 is not major risk but above 1000 can be risk for thrombosis (or bleeding if myeloproliferative disorder) especially if associated with cardiovascular risk factors.
Mild anaemia is common, and haemoglobin can fall with age. People generally tolerate a degree of anaemia. An Hb under 80 often causes symptoms needing investigation and treatment to reverse the cause. Between 80 and 100 may be acceptable but requires clinical assessment. Above 100 is usually asymptomatic. MCV helps tailor investigation to potential causes. Investigating isolated mild macrocytosis (1-2fL above range) is probably not helpful.
High Hb & PCV (Hct) may reflect reduced plasma volume (e.g. dehydration, obesity, alcohol smoking), secondary polycythaemia (hypoxic drive) or primary polycythaemia. Haemoglobin above 200 and PCV (Hct) over 0.60 requires urgent assessment.
Key messages
- Results slightly outside the reference range are common and may not be abnormal
- The further from the range, or association with symptoms or signs, and multiple abnormalities is more concerning.
Questions? Contact the editor.
Author competing interests: Nil.
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